1. Field of the Invention
The present invention relates generally to the field of brazing aerostructure panels and more particularly, but not by way of limitation, to an improved method and tooling arrangement for diffusing braze weight pressure in the brazing of a lightweight aerostructure honeycomb sandwich panel.
2. Description of the Prior Art
In the construction of certain aerostructures it is often necessary to provide metallic honeycomb sandwich panels which may then be secured together in a suitable manner to provide the ultimate aerostructure article. Such metallic honeycomb sandwich panels are often formed by brazing a central metallic honeycomb core layer to upper and lower face sheets. Such panels may have a configuration that is flat or which may have a certain desired curvature in their outer surfaces. Since such aerostructure articles may be subject to elevated temperatures, the panels may be constructed of metals such as Inconel and titanium among others and may also be relatively thin in thickness. Thus, a thin lightweight honeycomb sandwich panel is commonly constructed of thin sheets of Inconel or titanium and the honeycomb core layer is also fabricated of thin sheet stock.
It is common to fabricate such lightweight metallic honeycomb sandwich panels by brazing the opposing face sheets to the central honeycomb core layer in a furnace which subjects such panel to an elevated temperature over a period of time in a predetermined braze cycle. During such braze cycle it is essential to apply a uniform pressure to such face sheets to achieve a complete brazing of such face sheets to the edges of the honeycomb layer without experiencing a disbond condition at certain edges of the honeycomb layer or at the edge of the panel. Various arrangements have been proposed in the past to ensure that a uniform pressure is applied to such face sheets of the aerostructure panel. The arrangement that has found most favor in the art involves the use of a weighted pan that is positioned upon the upper face of the panel during the braze cycle to apply pressure to such panel assembly. Such a weighted pan, termed a "flexweight pan" in the art, is commonly constructed with a thin foil, for example 0.003" in thickness, as the bottom panel of the pan. When the pan is filled with a suitable weight such as tungsten pellets the pan will apply a pressure through the foil bottom panel which will conform to the surface configuration of the upper face sheet of the panel assembly. While an attempt is made to uniformly distribute the tungsten pellets over the thin bottom of the pan, nonetheless, because the face sheets of the panel assembly are so very thin, often in the neighborhood of 0.006", the distribution of such weight upon such face sheet is not applied uniformly to the face sheet and localized "heavy spots" are produced on such face sheet. These heavy spots can locally deform the thin face sheet of a panel assembly and create intra-cellular dimpling of the face sheet over the contacting edges of the honeycomb core layer. On the other hand, "light spots" may be produced on such face sheet of the panel assembly and cause a lack of intimate contact between the face sheet and the layer of honeycomb core thereby creating a disbond between such face sheet and the honeycomb core layer when the panel assembly is subjected to a braze cycle.
Also, differences in the coefficient of thermal expansion (CTE) between the material of the bottom panel of the pan and the materials of the face sheets of the panel assembly can also contribute to unacceptable wrinkling of a face sheet. Attention has been given to overcoming this CTE problem by interposing multiple layers of thin metal sheets, termed "slip sheets" between the bottom panel of the weighted pan and the face sheet of the panel assembly and lubricating such slip sheets with suitable lubricants termed "stop-off" compounds. In addition, a very slow time consuming heat up rate of the furnace for the ultimate brazing temperature of the brazing cycle is imposed. Thus, the expense of operating the furnace in which such a brazing operation is performed is significant because of the extensive time required to complete such a brazing cycle. Unfortunately, such a slow heat up rate to a brazing condition can also significantly adversely affect the properties of the face sheets and the braze material, as well, of the panel assembly. Thus, it is apparent that a present need exists in the state of the art today for an improved method and tooling arrangement for diffusing braze weight pressure in the brazing of lightweight aerostructure honeycomb sandwich panels and overcoming the shortcomings of the methods in present use.
Although the advances in techniques for brazing such aerostructures are note-worthy to one extent or another and are generally acceptable, none achieves the objectives of an efficient, reliable, inexpensive method of providing tooling to provide a uniform pressure to lightweight aerostructure panel assembly to be brazed that permits a brazing cycle to be completed in a minimum time period.
It is a general object of this invention to provide a method of tooling for the brazing of a lightweight aerostructure honeycomb sandwich panel assembly that provides a uniform pressure to such panel assembly during the braze cycle to preclude wrinkling of the face sheets of the panel assembly during the braze cycle.
It is a further general object of the invention to reduce the cost of the brazing cycle by providing a tooling arrangement that permits the braze cycle to be completed more quickly while also increasing the quality of the brazed panel assembly.
It is a further general object of this invention to provide an improved method of tooling for brazing a honeycomb sandwich panel that is less expensive in use, provides a superior brazed panel, and provides ease in use.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiments in addition to the scope of the invention defined by the claims taken in conjunction with the accompany drawings.